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THE THERMAL SAFETY UNDERSTANDING OF MXENE ANODES IN LITHIUM-ION BATTERIES
Rechargeable lithium ion batteries (LIBs) are widely used in various daily life applications including electronic portable devices, cell phones, military applications, and electric vehicles throughout the world. The demand for building a safer and higher volumetric/gravimetric energy density LIBs has increased exponentially for electronic devices and electric vehicles. With the high energy density and longer cycle life, the LIBs are the most prominent energy storage system for electric vehicles. Researchers are further exploring for new materials with a high specific capacity, the MXene has been a promising new anode material for LIBs. The typical MXene material Ti3C2Tz has 447mAh/g theoretical capacity, which is higher than traditional graphite (372 mAh/g for LiC6) based anode.
Though LIBs are used in most of the portable energy storage devices, LIBs are still having thermal runaway safety concern, which is caused by three main reasons: mechanical, electrical, and thermal abuse. The thermal runaway is caused by the initiation of solid electrolyte interface (SEI) degradation above 80 °C on the anode surface, generating exothermic heat, and further increasing battery temperature. The SEI is a thin layer formed on anode due to electrolyte decomposition during first few charging cycles. Its degradation at low temperature generates heat inside the LIBs and triggers the thermal runaway. The thermal runaway follows SEI degradation, electrolyte reactions, polypropylene separator melting, cathode decomposition and finally leads to combustion. The thermal runaway mechanism of graphite, which is the most common and commercialized anode material of LIBs, has been studied for years. However, the thermal safety aspects of the new MXene material has not been investigated yet.
In this thesis, we primarily used differential scanning calorimetry (DSC) and specially designed multi module calorimetry (MMC) to measure exothermic and endothermic heat generated at Ti3C2Tz anode, associated with multiple chemical reactions as the temperature increases. The in-situ MMC technique is employed to study the interactions and chemical reactions of all the components (separator, electrolyte, cathode and MXene anode) in the coin cell for the first time, while the ex-situ DSC is used to investigate the reactions happened on anode side, including electrolyte, PVDF binder, MXene, SEI and intercalated Li. Along with other complementary instruments and methods, the morphological, structural and compositional studies are carried out using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), Brunauer-Emmett-Teller (BET) surface area measurement and electrochemical measurement to support the thermal analysis. The electrochemical and thermal runaway mechanism of conventional graphitic anode is studied and used for comparison with MXene anodes.
The Ti3C2Tz thermal runaway is triggered by SEI decomposition around 120 °C analogous to conventional graphite. The thermal behavior of Ti3C2Tz anode is highly dependent on electrode material, surface area, lithiation states, surface morphology, structure and surface-terminating functional groups on Ti3C2Tz, which provides more active lithium sites for exothermic reactions with the electrolyte. Especially the terminal groups (-OH, -F, =O, etc.) from the etching process affect the lithium ion intercalation and thermal runaway mechanism. With annealing treatment, the surface-terminating functional groups are modified and can achieve less exothermic heat release. By normalizing the total heat generation by specific capacities of the anode materials, it is observed that Ti3C2Tz (2.68 J/mAh) generates slightly less exothermic heat than graphite (2.72 J/mAh) indicating slightly safer nature of Ti3C2Tz anode. The in-situ thermal analysis results on the Ti3C2Tz half-cell exhibited less total heat generation per mass (1.56 kJ/g) compared to graphite (1.59 kJ/g) half-cell.